1. Field of the Invention
The advantages of the present invention were discovered while working to obtain both primary and secondary fill-in data for the process disclosed in U.S. Pat. No. 4,619,684, Salladay et al., Oct. 28, 1986. The investigators who reduced the present invention to practice performed the original small pilot-scale pipe-reactor tests, as disclosed by Salladay et al. supra, at pressures up to 75 pounds per square inch gage (psig). As a follow-up to this work, pilot-plant granulation testing was requested, but the initial tests were not encouraging inasmuch as pipe-reactor pressures above about 45 psig caused significant disruption of the rolling bed of solids in the granulator with resulting high recycle ratios and ammonia evolutions. In view of these observed difficulties, it became obvious that additional improvements would have to be made to this process in order to provide that it could operate consistently to produce a high-quality product. Open aperture slurry distributors including open-end, slotted, and drilled-hole pipe distributors were all tested without success at pressures above about 45 psig, i.e., all of the distributors effected substantial disruption of the materials comprising the rolling bed. In addition, the small pilot-scale testing, referred to supra, was conducted with the pipe reactor operating at only one higher pressure, i.e., 75 psig and one lower pressure, i.e., 12 psig. In our investigations, it was determined that the data from said earlier tests were inconclusive and somewhat erratic and; accordingly, an additional set of tests was completed. The results of this additional set of tests did not correlate well with the initial set. In view of these results, it was decided that small-scale testing with the pipe reactor only should be repeated but with more comprehensive coverage of the pressure range and, more particularly, by extending the pressure range up to 140 psig. Details of this testing are given in greater detail in Example I infra and show that some different conclusions may be drawn which are different from the conclusions resulting from earlier tests because of more comprehensive data points in these latter tests. For instance, in initial small pilot-scale tests, the pipe-reactor pressures were adjusted with a manual diaphragm valve installed on the end of the reaction tube; this method was continued for the later comprehensive tests and the pipe reactor was tested and consistent data obtained for operating pressures up to 140 psig. However, the slurry was discharged from a turned-down, 90-degree elbow fitting installed on the discharge side of the valve and it had already been demonstrated this was not suitable for distributing the slurry into a granulator. A second phase of the testing was therefore begun to test various types of slurry distributors. The valve was removed from the discharge end of the reaction tube and various types of open-aperture distributors tested. These included an open-ended, turned-down elbow; an elbow with bushings inserted to decrease the cross-sectional area for flow; slotted pipe distributors; fan spray nozzles of various spray angles ranging between about 50 to about 90 degrees which effected a flat fan-shaped spray pattern; hollow-cone as well as solid-cone spray nozzles, both producing a circular spray pattern; and flooding nozzles that produced flat 120- to 140-degree, wide-angle spray pattern. Since the initial small pilot-scale testing included only operation of a pipe reactor and not any granulation testing, all slurry distribution systems could be readily tested at pressures up to 140 psig. The small-scale test data showed results that were not readily apparent in previous work that studied only one moderate pressure of about 75 psig. For instance, in these later tests it was discovered that, contrary to Salladay et al.'s interpretation of the theoretical data that ammonia evolutions from the pipe reactor decrease with increasing pressure, the ammonia evolution from the pipe reactor increases rapidly with increasing pressure; but only up to a certain point, i.e., about 75 psig. Quite unexpectedly, the ammonia evolution remained relatively constant from about 75 up to about 140 psig as evidenced by the significantly decreased slope of that portion of the curve comprising FIG. 4 above about 80 psig which will be discussed in greater detail infra. Increasing ammonia evolution with increasing pressure is not surprising since, as shown in Example I infra, for otherwise constant conditions and with a constant phosphoric acid feed and concentration the operating temperature of the pipe reactor increases with increasing operating pressure as might be expected by the ideal gas law and by the increasing temperature of the steam as water evaporates from the ammoniated phosphoric acid slurry. As a result of the increased temperature, the ammonia partial pressure increases at a faster rate than the operating pressure so that free ammonia evolving from the discharge of the pipe increases, but it was somewhat surprising that seemingly at about 75 psig the rates of increased partial pressure and operating pressure were such that the ratio of ammonia partial pressure to operating pressure stopped increasing and reached a relatively stable ratio resulting in an almost constant ammonia evolution. This was not as predicted by Salladay et al. because their theoretical graphical illustration of ammonia versus NH.sub.3 :H.sub.3 PO.sub.4 mole ratio for the ammonia, phosphoric acid, and water system was plotted only at 250.degree. F. As shown in our Example I infra, a constant temperature is not obtained as pressure increases. A constant temperature could be obtained by increasing the water content or similarly by decreasing the phosphoric acid concentration; however, this would be impractical for making a fertilizer product because of loss of process control and substantial increase of required drying heat.
Based on the knowledge gained from the small pilot-scale results, further large pilot-scale granulation tests were performed which tests used higher pipe-reactor pressures and improved flooding spray nozzles as slurry distributor means. It should be pointed out here that, although ammonia evolutions from the pipe reactor are higher at higher pressures, only a portion of the ammonia to the process is fed to the pipe reactor, i.e., about 65 to 70 percent, with the remainder to the bed of solids in the drum granulator. Since there are two sources of ammonia evolution, the pipe reactor and the bed of solids in the granulator, the important point to remember is that the total amount of ammonia evolved is from the granulator from both sources and their interaction is important. Pilot-plant data showed, as discussed in greater detail in Example II infra, that although ammonia evolved from the pipe reactor would be expected to increase with increasing pressure, the total ammonia evolved from the granulator is surprisingly decreased. Salladay et al. surmised this was because the injection of free ammonia as a high-pressure stream of discharged slurry impacts the bed of solids. Although this may be partially true, even with the high-velocity, high-impact slotted distributor presently used in commercial-scale plants, severe disruption of the bed occurs at only 60 psig and even as low as 45 to 50 psig which disruption impedes process control. For otherwise comparable tests, which can be well controlled on pilot scale but which would be difficult and potentially very expensive to set up and control on commercial-scale plants, the low-impact slurry spray distributor resulted in lower overall ammonia evolutions than did the high-impact distributor used by Salladay et al. at pressures of about 75 psig. In addition to less bed disruption, far superior slurry spray coverage was obtained, in the practice of the instant invention. This is an important consideration when subsequently ammoniating the resulting solids to make a homogeneous fertilizer product. Surprisingly, in the practice of the present invention lower temperatures were maintained in the drum granulators than were observed in the practice of the teachings of Salladay et al. Since ammonia vapor pressure over ammonium phosphate solutions is directly related to temperature, in the operation of the present invention lower overall ammonia evolutions were obtained from the granulator. We also discovered that better cooling, most likely evaporative-type cooling, occurs as the high-pressure slurry produced in the pipe reactor is discharged and effects the evaporation of water as the pressure is lowered to atmospheric pressure within the drum granulator. We believe that this cooling is more effective in lowering ammonia evolution from the granulator than any ammonia injection effect.
2. Description of the Prior Art
In U.S. Pat. No. 3,153,574, Achorn et al., Oct. 10, 1964, assigned to the assignee of the present invention, a process for granulation of granular diammonium phosphate fertilizer from wet-process phosphoric acid and ammonia is taught whereby a large preneutralizer tank is required to partially neutralize the acid. This conventional process uses the TVA ammoniator-granulator described in U.S. Pat. No. 2,729,554, Nielsson, Jan. 3, 1956, who teaches a process for ammoniating superphosphate and not for production of diammonium phosphate. See specifically, but not necessarily exclusively, FIG. 3 of U.S. Pat. No. 2,729,554 supra, and the discussion thereof as same relates to the alternately rising and falling motion of the material within the bed. The present invention has the distinct advantage of eliminating the need for the preneutralization step and the large preneutralizer tank of such prior art teachings. The process of the instant invention also eliminates any need for a pressurization step as well as the requirement for special equipment, such as a dehydration chamber, as taught in U.S. Pat. No. 3,415,638, Hemsley et al., Dec. 10, 1968. No complicated acid preparation step, as taught in U.S. Pat. No. 2,902,342, Kerley, Sept. 1, 1959, nor a slurry separation step before granulation, as taught in U.S. Pat. No. 3,310,371, Lutz, Mar. 21, 1967, is required in the practice of the present invention.
In U.S. Pat. No. 3,954,942, Achorn et al., May 4, 1976, assigned to the assignee of the present invention, there is mentioned the production of granular diammonium phosphate in a pipe-cross reactor but no specifics of operation disclosed. In U.S. Pat. No. 4,134,750, Norton et al., Jan. 16, 1979, also assigned to the assignee of the present invention, therein is disclosed pilot-plant operating data for production of granular diammonium phosphate with a pipe-cross reactor. Unike the teachings of U.S. Pat. No. 4,134,750 supra, the practice of the present invention requires no sulfuric acid as feed to the reactor; in this case a simpler pipe reactor, to reduce reactor scale buildup. This elimination of the requirement for sulfuric feed is a significant improvement especially when the quality of the feed phosphoric acid is so poor that maintaining the nonimal 18-46-0 grade of a diammonium product is difficult. When sulfuric acid is fed to a granular diammonium phosphate process, it acts as a diluent which furnishes no major fertilizer nutrient value and decreases the value of the major fertilizer nutrient P.sub.2 O.sub.5 . In both of the Achorn et al. processes and the Norton et al. process supra a reactor is used which operates at relatively low pressures, i.e., 40 psig or less. The present invention uses a reactor operating at much higher pressures which provides the advantage of lower overall ammonia evolutions and recycle ratios. These advantages will be described in greater detail in several of our Examples infra.
In U.S. Pat. No. 4,427,433, Parker et al., Jan. 24, 1984, assigned to the assignee of the present invention, there is described an energy-efficient process for producing an ammonium polyphosphate granular fertilizer and by which a predictable polyphosphate could be obtained. The process uses a pipe reactor and drum granulator and other equipment similar to that used in the practice of the instant invention. Indeed, it can be shown that the configuration of the pipe reactor is remarkably similar to the reactor of the present invention. However, the polyphosphate product was made at extremely high temperatures, i.e., over 400.degree. F. and had an NH.sub.3 :H.sub.3 PO.sub.4 mole ratio of only about 1.05:1. This process of the prior art was not suitable to produce a higher NH.sub.3 :H.sub.3 PO.sub.4 mole ratio product such as diammonium phosphate, which is ammoniated to a mole ratio of 1.9:1 to 2:1, because of excessive ammonia evolutions at such higher mole ratios, temperatures, and solution concentrations required to produce a polyphosphate-containing product. It has now been determined in the investigations leading to the instant invenion that the reactor used in the Parker et al. process supra can be adapted to produce diammonium phosphate products with judicious adjustment of process parameters and use of our improved distributor. This reactor has advantages over the extremely long-inclined reactors used in the Salladay et al. process discussed supra.
In U.S. Pat. No. 4,601,891, McGill et al., July 22, 1986, assigned to the assignee of the present invention, there is taught a process for producing an ammonium polyphosphate granular fertilizer using a pipe reactor spraying an ammonium polyphosphate melt through fan spray nozzles at about 50 psig onto a falling curtain of solids in a rotating drum granulator. Again this process cannot be effectively utilized to produce a diammonium phosphate fertilizer because, although the reactor might produce and spray the appropriate slurry, the more complicated flighted drum granulator is not conducive to having the required additional ammonia added to the bed of solids because such flights do not allow the necessary submerged sparger to be used. A separate ammoniation step would be necessary thereby resulting in more equipment and higher construction and operating costs. Since diammonium phosphate is produced in a highly-competitive, commodity-type market, it is imperative that simple and minimum-cost equipment and processes be used.
In U.S. Pat. No. 4,619,684, Salladay et al., Oct. 28, 1986, supra, assigned to the assignee of the present invention, there is discussed a process for producing diammonium phosphate which uses similar equipment and operating parameters to that of the instant invention. The present invention might even be considered a continuation of Salladay et al.'s work since the investigators of the present invention performed the initial small pilot-scale testing of the Salladay et al. process as given in their Example I, and the present invention resolves some difficulties of the Salladay et al. process while adding additional substantial improvements thereto. Salladay et al. claim operation of a reactor within the range of 40 to 80 psig although their Example III, which shows results of testing in a commercial plant, discloses only 40 psig and; although their Example I does show reactor operation at 75 psig, this was a test of reactor operation only, i.e., they did not distribute the reactor slurry into a granulating vessel. As a result, conditions in their commercial plant test were such that pressures much above about 40 psig resulted in the bed of solids in the granulator being so blown about and deformed by the high pressure and high-velocity gases and slurry stream from the reactor that consistent operation could not be sustained. Subsequent follow-up testing by the present investigators in a granulation pilot-plant facility verified the difficulties of distributing a high-pressure stream from the reactor without severe deformation of the granulator bed of solids and resulting degregation of resulting product. Accordingly, we performed additional testing to correct this problem and discovered that one type of special distributing nozzle, one which had at times been used to apply fertilizer solutions onto fields in farm applications but which had not been used either to distribute ammonium phosphate slurries as in a plant producing diammonium phosphate or to distribute similar products at rather high pressures, i.e., 80 psig and higher, was most suitable. The test work leading to the discoveries of the instant invention and described further in greater detail in the Examples below showed that a spray nozzle commonly referred to as a flooding nozzle allowed excellent slurry distributions at 80 psig reactor pressures and even at pressures as high as 140 psig. This nozzle provided the advantages of a low-velocity and low-impact slurry spray and a wide angle of dispersion, i.e., about a 130-degree arc to cover a large area with each nozzle while allowing the nozzle to be spaced substantially closer to the bed of solids in the granulator which greatly simplifies any retrofit installation because space is usually a premium not only in existing plants, but also in newly planned installations. Our data also show that the use of this slurry distributor resulted in lower granulator temperatures for otherwise comparable conditions and lower resulting ammonia evolutions from the granulator. These ammonia evolutions are even lower than those reported in Salladay et al.'s process, which indicates that a high-pressure, high-velocity spray that injects the free ammonia into the moist bed of granules is not the best mechanism to result in lower ammonia evolutions and that the extra evaporative cooling resulting from the present invention's special spray distributor is more significant since this spray is a low-impact one which gives an excellent and uniform distribution of slurry especially at high pressures. We, of course, are aware of the slotted aperture distributor used by Salladay et al. and are also aware that it can be useful since we were, during our pilot-scale testing, the first to use a slotted distributor with a pipe or pipe-cross-type reactor to produce granular fertilizers. We are also aware of the limitations of this type of slotted distributor which limitations include a poorer, less uniform spray pattern and higher spray velocity and higher spray impact than the distributor of the present invention, especially when operated at higher pressures contemplated in the instant invention.
Salladay et al. supra also use a long (over 35 feet long plus slurry distributor section) reactor with a specially designed ammonia sparger with a series of drilled holes that must be specially designed and sized to allow uniform pressure dispersion and distribution of the ammonia feed. The reactor of the present invention, as shown in FIG. 2 infra, and described in greater detail infra is of a different design which requires less space for installation. As has been noted supra, this is an extremely important consideration since space is a precious commodity if this process is to be effectively retrofitted to an existing plant. The reactor means of the present invention allows the initial reaction to be carried out in a vertical orientation and at a relatively low velocity in a pool of partially ammoniated acid slurry with a high residence time to thereby improve ammonia retention of the acid and allow good internal separation of the gas-liquid phases. The horizontal section which is just downstream from said vertical section, is sized to yield higher velocities of slurry movement and allow vertical separation of the gas and liquid phases to be maintained while minimizing the retention time with resulting minimization of unit heat losses which, with the intimate mixing of liquids in the vertical section and vertical to horizontal transition section, result in no material buildup problems. In addition, because the reactor of the present invention has an initial vertical section, the ammonia sparger is only a straight open-end pipe which effects very uniform ammoniation of the phosphoric acid and alleviates the need for a specially designed ammonia sparger as required with Salladay et al.'s long, horizontally inclined reactor.
For ease in comparing the advantages and attributes of the instant invention, especially the reactor slurry distributor, but also higher operation pressures over the prior art, particularly that taught by Salladay et al., following is a tabulated summary of pilot-plant operation results from process operating parameters taught by the instant invention versus those of the Salladay et al. process for otherwise comparable operating conditions. These pilot-scale tests were performed to allow testing of the two processes under comparable operating conditions in the same plant so that an accurate comparison of the processes could be made. Note especially the lower ammonia evolution and drying requirements of the present invention as well as the lower recycle ratio which results in higher production rate capability. The lower recycle ratio is partially due to lower temperatures obtained under otherwise comparable conditions but is principally attributed to the low-impact slurry distributor utilized in the instant invention thereby allowing high-pressure operation of the pipe reactor without severe disruption and blowing away of portions of the solid bed of recycle solids in the granulator.
______________________________________ Instant Salladay Item invention et al. '684 ______________________________________ Pipe reactor Operating pressure, lb/in.sup.2 g 73 68 Temperature, .degree.F. 337 347 Slurry NH.sub.3 :H.sub.3 PO.sub.4 mole ratio 1.39 1.43 Granulator Temperature, .degree.F. 176 186 Recycle ratio, lb/lb product 1.8 5.0 Ammonia evolution, % 13 16 Dryer heat, Btu .multidot. 10.sup.5 /ton product 0 2.7 Product NH.sub.3 :H.sub.3 PO.sub.4 mole ratio 1.94 1.91 ______________________________________